initcalls: Add initial implementation

Create simple serial subsystem which makes use of initcalls, and convert
existing serial drivers to its use.

kernel/console.c

@@ -123,17 +123,17 @@ void console_newline() {
 	}
 }
 
-void console_putc(char c) {
+int console_putc(char c) {
 	char *console_buffer_end = console_buffer + chars_per_line * lines;
 	char *write_char_at;
 
 	switch (c) {
 		case '\n':
 			console_newline();
-			return;
+			return 0;
 		case '\r':
 			curr_char = 0;
-			return;
+			return 0;
 		default:
 			break;
 	}
@@ -148,4 +148,6 @@ void console_putc(char c) {
 	*write_char_at = c;
 
 	console_char_to_fb(c, curr_char++, curr_line);
+
+	return 0;
 }